Its back was armoured, and it is placed in the Thyreophora with the other armoured dinosaurs. It was one of the earliest members of this group, and the most basal (primitive) form so far known. Its closest relatives may have been Emausaurus and Scelidosaurus. These, and the larger armoured dinosaurs Ankylosaurus, Stegosaurus and Huayangosaurus, were all quadrupedal. The lightly built bipedal Scutellosaurus is similar in form to the ancestral dinosaurs, which were bipedal.[1]

Scutellosaurus was around 1.2 metres (3.9 ft) long,[2] 50 centimetres (20 in) tall at the hips, and weighed 10 kilograms (22 lb). The fossil evidence includes two partial skeletons recovered from Arizona, although of the skull only the lower jaw has been recovered. There were also several hundred scutes running along its neck to its back and as far down as its tail. Some of these shields were flat, while others were pitted. It had an unusually long tail, possibly a counterbalance to the weight of the armoured body, and long arms that suggest it may have browsed on all fours.[2]

↑The dinosaurs were ancestrally bipedal: all the earliest ones known were bipedal. That means the transition from a quadrupedal archosaur reptile to a bipedal form took place before the dinosaurs evolved. The heavier dinosaurs later became quadrupedal again. This was, for them, a secondary or derived condition.